Uranium enrichment
Uranium enrichment describes the various processes used to increase the proportion of the isotope 235 U in uranium . Natural uranium consists of about 99.27% of 238 U and 0.72% of 235 U. Uranium enriched in different levels of 235 U is used as a nuclear fuel for nuclear reactors and nuclear weapons . Enrichment is a branch of the uranium economy .
Procedure
In a uranium enrichment plant, natural uranium (“ feed ”) is separated into two fractions, one of which (“ product ”) has a higher proportion of 235 U than the starting material, the other (“ tails ”) a lower proportion . The work performed by a separation device is expressed in kilograms of uranium separation work (kg UTA) or tons of uranium separation work (t UTA). In the English specialist literature, this unit is referred to as the SWU ( Separative Work Unit ).
The common industrial processes use uranium hexafluoride (UF 6 ) as the process medium , the only chemical compound of uranium that has sufficient volatility for the separation process at room temperature (approx. 100 mbar vapor pressure at room temperature). First, yellow cake is extracted from the uranium ore by leaching , a mixture of different uranium compounds, predominantly oxides. Uranium hexafluoride is made from the yellow cake and processed using the gas diffusion method or gas centrifuges.
Uranium hexafluoride is also so well suited for the enrichment process because fluorine only occurs in nature as a pure element ( isotope fluorine -19). The mass of the UF 6 molecules therefore only varies due to the different masses of the uranium isotopes. Because of the small mass of the fluorine atom, the relative mass difference between the UF 6 molecules is still around 0.85% compared to around 1.3% relative mass difference between the uranium isotopes themselves:
Use of the enriched uranium
235 U is - like some other nuclides with an odd number of neutrons - relatively easily fissionable by thermal neutrons and the only known naturally occurring nuclide that is capable of a nuclear fission chain reaction . While natural uranium can also be used for heavy water and graphite- moderated reactors, the more common light water reactors must be charged with uranium, the 235 U content of which has been increased to at least 3% - in practice up to 5%. As highly enriched uranium ( " HEU " from English highly enriched uranium ) uranium having 20% or more is 235 U respectively. Very high enrichment is required for nuclear weapons (typically at least 85%).
Use of the depleted uranium
The by-product of the enrichment is depleted uranium. For each ton of nuclear fuel enriched for civil purposes, around 5.5 tons of depleted uranium with a 235 U content of around 0.3% are produced. Because of its high density, it is used in counterweights for aircraft wings and racing yachts, as well as in uranium ammunition for military purposes. For such purposes, however, only about 5% of the accumulated depleted uranium is used, the rest is stored. The main interest in this material, especially on the part of Russia , is its use as a mixed material (“blender material”) for the reconversion of highly enriched (military) uranium into weakly enriched (civil) uranium for use in light water reactors. The disarmament campaign under the START II agreement should be mentioned here in particular : “Megatons to megawatts”. According to the Atomic Energy Act of the Federal Republic of Germany , depleted uranium is a valuable material .
Methods
In terms of efficiency, enrichment using gas centrifuges is superior to the diffusion method by a factor of around 10, and laser enrichment by a factor of around 2 to 20.
Enrichment by gas centrifuges
The gas centrifuge process is now the more common process for uranium enrichment in the international field and has meanwhile overtaken gas diffusion in its importance. The most important reasons for this are the considerably lower energy consumption (around 50 kWh per kg UTA; for comparison: diffusion separation up to 2500 kWh per kg UTA) and greater flexibility in capacity planning.
In the gas centrifuge process, gaseous uranium hexafluoride (UF 6 ) is fed into the interior of a vertical cylinder that rotates very quickly (> 60,000 / min) . Under the influence of the high speed and the resulting mass-dependent centrifugal force , the heavier 238 UF 6 molecules accumulate on the inner wall of the cylindrical rotor and the lighter 235 UF 6 molecules near the rotor axis, so that the isotopes can be removed separately.
The separating effect is enhanced in modern centrifuges by generating an axial circulating heat flow by heating the lower part and cooling the upper part of the centrifuge. Such centrifuges are also known as countercurrent centrifuges . The greatest difference in mass between the mass flow enriched and depleted with 235 U is no longer between the axis and the rotor wall, but between the ends of the centrifuge. The enriched, light fraction (“ Product ”) is removed from the upper (cold) end, the depleted, heavier fraction (“ Tails ”) from the lower (warm) end of the centrifuge.
The extraction tubes for the enriched and depleted fraction protrude into the area of the rotating gas on the inner wall of the centrifuge and thus use the dynamic pressure to transport the gas within the system. The separation process takes place under negative pressure, so “ Product ” and “ Tails ” must be brought to normal pressure with the help of compressors and sublimers / desublimators before they can be filled into transport or storage containers.
The gas centrifuges are usually connected in cascades with several hundred individual centrifuges, since each centrifuge can only achieve a limited throughput and a limited enrichment. The parallel connection of the centrifuges increases the throughput, while the enrichment is increased by connecting them in series. The efficiency of the centrifuges can be increased by increasing the tube length and, in particular, the speed of rotation; they therefore have an elongated, roller-like shape. With aluminum alloys , 400 m / s are achieved, with high-strength steels 500 m / s and with fiber-reinforced materials over 700 m / s. The separation performance is practically limited by the material properties of the rapidly rotating rotor as well as by technical limitations of the rotor length (occurrence of undesired natural vibrations).
Diffusion methods
In the gas diffusion method , gaseous uranium hexafluoride (UF 6 ) is allowed to diffuse through a porous membrane . The driving force here is the pressure difference on both sides of the membrane. Molecules containing 235 U are lighter than those containing 238 U and diffuse faster. In the case of a uranium isotope mixture, the gas flow that diffuses through the pores in the wall (“ product ”) therefore contains a slightly higher proportion of the isotope 235 U than the original flow (“feed”). A single separation stage has a low separation factor (concentration ratio of the 235 U in Product and Tails) of a maximum of 1.0035, but a high material throughput. For a degree of enrichment that is sufficient for the operation of light water reactors , around 1200 stages connected in series are required, which together form a so-called "cascade". The energy consumption is high and is around 2300-2500 kWh per kg uranium separation work (UTA).
Instead of the pressure difference, a temperature gradient can also be used to separate isotopes by means of diffusion (see thermal diffusion ). However, these processes play no practical role in uranium enrichment.
Electromagnetic enrichment
As in a mass spectrometer , in electromagnetic isotope separation, uranium atoms are first ionized , then accelerated in an electric field and then separated in a magnetic field according to the different mass numbers. This isotope separation setup was used in World War II to produce enriched uranium for the first atomic bombs; the systems used at that time were called calutrones .
Because of the enormous effort involved, this process is no longer of any importance for the production of enriched uranium. However, it is used in research for other isotope separations, since ideally a single obtained atom of an isotope can be detected.
Laser process
The laser process is based on the isotope shift of the absorption spectra of atoms and molecules. Are the spectroscopic conditions suitable, i. H. if the absorption lines of the isotopes or isotope compounds overlap sufficiently little, and if a laser of suitable wavelength and narrow band is also available, an isotope-selective excitation is possible. The separation takes advantage of the fact that the excited species differs significantly from the non-excited species in terms of their physical and chemical properties. Laser processes are characterized by a high level of selectivity.
Basically, two concepts can be distinguished: the photoionization of uranium vapor (atomic process; AVLIS ) and the photo-dissociation of UF 6 (molecular process; MLIS ). Theoretically, the laser process allows isotope separation in a single step. In practice, the number of stages required depends on the extent to which the ideal conditions can be achieved.
In the atomic process, the atoms of an isotope mixture are selectively ionized. After ionization of one isotope ( 235 U) it can easily be separated from the non-ionized atoms of the other isotope ( 238 U) by acceleration in an electric field.
In the SILEX process , which was developed back in the 1970s, the molecules of the gaseous uranium hexafluoride containing 235 U should be selectively excited by a first laser ( e.g. a frequency-stabilized carbon dioxide laser ) before a fluorine atom is split off by a second laser . The resulting solid 235 UF 5 can easily be filtered out of the gas.
After initial euphoria about the advantages of these processes compared to conventional, established enrichment processes, people became skeptical about their industrial feasibility. Most countries withdrew from this technology because the technical problems (e.g. corrosion on the equipment) seemed insurmountable.
In the meantime, however, there have been developments for the large-scale application of this process. For example, a plant is currently being built near Wilmington in North Carolina that will enrich uranium using laser isotope separation from 2013. Critics warn against it, however, because it would make nuclear weapons production easier and less controllable.
Separation nozzle process
The separation nozzle process was also developed in Germany by the end of the 1980s . Here the segregation of the uranium isotopes takes place due to different centrifugal forces in a fast, curved flow. In 1975, Brazil adopted this process as part of the German-Brazilian nuclear energy agreement in order to process its large uranium deposits; however, the planned systems were not implemented. As an advantage of the separation nozzle process came here u. a. to ensure that it was not subject to any confidentiality restrictions. The Republic of South Africa practically used the separating nozzle method before 1990, as the embargo against the country meant that only technologies could be used that could be used without great difficulty (without confidentiality restrictions by the Federal Republic of Germany ). The high energy consumption was accepted.
Enrichment capacity
Diffusion plants still have the largest share of the total accumulation capacity installed worldwide . However, the production share of centrifuge systems is increasing due to the technical dominance of advanced gas centrifuges. In France , the existing gas diffusion system (Georges Besse I) will soon be replaced by a modern centrifuge system (Georges Besse II). Two new centrifuge plants are planned in the USA.
The total amount of highly enriched uranium worldwide in 2010 was approximately 1580 tons. Large plants have an annual capacity in the range of several 1000 t UTA.
The following table gives an overview of the most important existing plants (with capacities over 100 t UTA / a):
| country | investment | operator | Procedure | UTA tons / year | |
|---|---|---|---|---|---|
| current | Final expansion | ||||
| China | at Lanzhou | CNNC | diffusion | about 700 | |
| China | at Hanchong | centrifuge | 200 | ||
| Germany | Gronau | Urenco | centrifuge | 3,900 | 4,500 |
| France | Tricastin | Eurodif | diffusion | 10,800 | 07.2012 shut down |
| Great Britain | Capenhurst | Urenco | centrifuge | 4,200 | |
| Japan | Rokkasho | JNFL | centrifuge | 1,050 | 1,500 |
| Netherlands | Almelo | Urenco | centrifuge | 3,600 | |
| Russia | Krasnoyarsk | centrifuge | 2,500 | ||
| Russia | Rostov on Don | Techsnabexport | centrifuge | 1,400 | |
| Russia | Ekaterinburg | centrifuge | 10,000 | ||
| Russia | Angarsk | centrifuge | 1,400 | ||
| Russia | Tomsk | centrifuge | 5,700 | ||
| United States | Paducah Gaseous Diffusion Plant near Paducah | USEC | diffusion | 11,300 | Shut down in 2013 |
| United States | National Enrichment Facility near Eunice (New Mexico) | LES (Urenco) | centrifuge | 0 | 5,900 |
| total | 56,050 | ||||
Importance of uranium enrichment for the construction of nuclear weapons
Uranium enrichment is one of two avenues for building nuclear weapons. The other way is to incubate plutonium in a nuclear reactor and then separate it from the used nuclear fuel through reprocessing .
If the weapon is to achieve high explosive power, i.e. be of strategic military interest, then the relevant isotope, 235 U or 239 Pu, must be present in almost pure form in both cases . For nuclear weapons of lower effectiveness, but which z. B. would be interesting enough for terrorist groups, less pure 235 U or 239 Pu is sufficient .
The conventional chemical blasting technology required to ignite a nuclear weapon is less demanding for uranium than for plutonium (see nuclear weapon technology ). Because of the lower radiation, a uranium bomb is easier to store and easier to handle than a plutonium bomb.
An enrichment plant requires at least a comparable technological level for construction and operation as a reprocessing plant. To put it simply, it has to deliver kilograms with high enrichment for military weapons purposes, whereas for reactors for energy supply it has to supply tons with low enrichment. In the case of gas centrifuge technology, the fact that a system is only used for the latter purpose can only be ensured by constant or sufficiently frequent inspections, because in principle such a system can be converted from one to the other purpose by changing the pipeline connections between the centrifuges.
In August 2005, the global public looked at Iran and the controversial restart of its "nuclear complex" in Natans , Isfahan province . There, uranium enrichment is carried out to a comparatively small extent, the degree of enrichment achieved is far from being bomb-proof. Iran is claiming its right to enrichment for civil energy supply purposes. However, mastering gas centrifuge technology for enrichment is, as described, an essential step on the way to nuclear power. In February 2010, then President Mahmud Ahmadinejad declared that uranium would be enriched by up to 20%.
Web links
- Forschungszentrum Jülich - safety research and reactor technology at the Institute for Energy Research with an information library on nuclear energy
- Research Center Karlsruhe - nuclear safety technology and nuclear fusion at the energy research area of the Helmholtz Association with historical data on uranium enrichment and a lexicon on nuclear energy
- Urenco Germany - leading provider of services and technologies for uranium enrichment
Individual evidence
- ↑ Stop laser uranium enrichment (Nature 464, 32-33, March 4, 2010)
- ↑ Presentation to House of Representatives Standing Committee on Industry and Resources ( Memento of the original dated February 16, 2011 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (pdf, Global SILEX Systems Limited, February 9, 2006)
- ^ Uranium Enrichment
- ↑ JW Eerkens: Spectral Considerations in the Laser Isotope Separation of Uranium Hexafluoride , in: Applied Physics , 10/1976, pp. 15-31; doi : 10.1007 / BF00929525 .
- ^ Cheap fuel for nuclear power plants , Die Zeit , June 13, 1975, No. 25.
- ↑ Enriching uranium with lasers - A new system is to supply 42 million households in the USA with more compact, faster and cheaper electricity. In: Welt am Sonntag . dated August 28, 2011.
- ↑ PhysikKonkret No. 11, March 2012: SILEX Risk of Uranium Enrichment ( Memento of the original from July 24, 2013 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ False Lessons from the Cold War . In: Technology Review of January 4, 2012.
- ↑ Report on Australian television from August 1, 2013, accessed on August 2, 2013 (English)
- ^ Atomic Summit in Washington, article from Der Spiegel, April 12, 2010
- ↑ Urenco Germany | Urenco. Retrieved December 14, 2019 .
- ↑ Information to the public according to the Radiation Protection Ordinance and the Hazardous Incident Ordinance. 6. Repeat information. urenco, April 2019, accessed December 14, 2019 .
- ↑ Urenco-UK-Capenhurst ( Memento of the original from January 19, 2012 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ↑ Urenco NL Almelo ( Memento of the original from December 10, 2008 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice.
